Contradictory results in schizophrenia research are generally explained as being due to genetic heterogeneity and multiple factor heredity in relationship to manifold environmental factors. The book reports a short overview of all relevant twin studies on schizophrenic psychoses and provides data and case histories on a systematic twin study based on a polydiagnostic approach carried out by two independently working psychiatrists. In addition to the internationally applied operational diagnostic systems of DSM-III-R and ICD 10 Leonhard¿s subclassification of schizophrenic psychoses was used. Up to now this sophisticated methodological approach is unique in the world.The data provide strong evidence that the spectrum of psychoses with schizophrenic and schizophrenia-like symptoms is not a continuum of diseases. At least in Leonhard¿s three major groups of cycloid psychoses, unsystematic schizophrenias and systematic schizophrenias genetic, somatic and psycho-social factors play a completely different etiological role. Cycloid psychoses and systematic schizophrenias are predominantly caused by "environmental¿ factors. In unsystematic schizophrenias, however, genetic predisposition is the main etiological factor and environmental factors are subordinate.

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The diagnostic dilemma in psychiatry.- The nature - nurture controversy.- Methods of clinical genetics in psychiatry.- General remarks on twin origination.- Biology of twin origination.- Frequency of twin births.- The twins method in research.- Characteristics of the twin constellation.- The methodological basis of twin research.- Classical twin studies in schizophrenia research.- Severity of illness and concordance rates.- Concordance/discordance and familial pattern.- Concordance/discordance and handedness.- Concordance rate of dizygotic twins and other sibling morbidity rate.- Monozygotic twins reared apart.- Frequency of schizophrenia in twins.- Studies of monozygotic twin pairs discordant for schizophrenia.- Further questions.- Methodology of a systematic twin study.- Twin survey methodology and zygosity diagnosis.- Methodology of psychiatric diagnostics.- Determining the criteria for concordance/discordance.- Selection of further test variables.- Results.- Results of the twin survey and zygosity diagnosis.- Psychiatric diagnoses.- Demographic data of the subjects.- Concordance rates.- Case studies.- Summary of the family history.- Birth history in the intra-pair comparison.- Role assignment in the twin constellation.- Handedness of test persons.- Discussion.- Role assignment (dominant/subordinate) and handedness .....- Comparability of diagnoses.- Demographic data in polydiagnostic comparison.- Varying genetic dispositions.- The absence of schizophrenic subgroups among twins.- The influence of pre- and perinatal complications.- The role of prenatal developmental disorders.- Acknowledgements.